1. Field of the Invention
The present invention relates generally to a vacuum ionization gauging tube, and more particularly to the Bayard-Alpert (B-A) type vacuum ionization gauging tube to improve measurable capabilities and handling convenience.
2.Description of the Prior Art
It is known that the vacuum ionization gauging tubes are widely used to measure the pressures for a high- and ultrahigh-vacuum region. Typically, the conventional vacuum ionization gauging tube includes a hot-wire filament for emitting electrons, a helical grid having positive potential relative to the hot-wire filament for collecting electrons, and an ion collector having negative potential relative to the hot-wire filament for collecting positive ions. The electrons emitted from the hot-wire filament are being accelerated while traveling across the electric field developed by the grid, and are attracted to and collected by the grid. Some of those electrons collide against the gaseous molecules, therefore, producing positive ions which may be attracted to and collected by the negatively-biased ion collector. Those ions flow through the ion collector, therefore, ion current as an electric current value is measured by an ammeter externally connected to the ion collector. The measured electric current value is then converted to the corresponding pressure value.
There is a variety of vacuum ionization gauging tube types. One of them is known as the Bayard-Alpert (B-A) vacuum ionization gauging tube having the design as shown in FIG. 8. As seen from FIG. 8, this type of vacuum ionization gauging tube includes the filament, grid and ion collector as referred to above and which are arranged substantially coaxially such that the needle-like ion collector 23 is located on the axis of the tube structure and extends along the axis, the grid 22 is shaped like a spiral winding coil or a helical grid and surrounds the ion collector 23, and the filament 21 is disposed outside of the grid 22 and spaced away from it. There is another vacuum gauging tube that improves the previously described vacuum gauging tube in providing the capabilities for measuring the higher pressure range. This vacuum gauging tube further includes a supplemental or auxiliary electrode 24 that is disposed to surround the filament. An envelope 11 is usually made of glass, within which those electrode elements are arranged such that they can be hermetically isolated from the atmospheric pressure. In some types, the inner wall of the envelope 11 is coated with a metal film 25 which is grounded so that the inner wall of the envelope can be maintained a constant potential. The envelope 11 includes an outlet pipe 12 extending at right angles to a main portion of the envelope 11, and which is connected to a vacuum chamber system, container or the like being monitored from the outside.
FIG. 9 is a schematic diagram showing the potential gradient for each electrode shown in cross section. Those potentials are supplied by the external controlling power supply 31 connected to the ionization gauging tube. Specifically, the ion collector 23, the supplemental or auxiliary electrode 24, and the metal film coating or lining 25 are at the ground potential, whereas the filament 21 is biased positively and the grid 22 is at a higher positive potential. The potential difference between the filament and grid is usually set to about 150 volts which attains the maximum ionization efficiency for most gases. The thermoelectrons that are emitted from the hot-wire filament 21 are attracted by the grid 22 at the positive potential, being accelerated while traveling toward the grid 22. The electrons that have passed the grid 22 are influenced by an electric field developed by the ion collector, and are thus being decelerated and repelled back to the grid 22. In this manner, those electrons may have oscillations while cycling about the gird 22. Some of the electrons may collide against the gaseous molecules, producing positive ions. Those ions are attracted by the electric field across the ion collector 23, and are collected by the ion collector 23. The ions are measured as ion current within the externally-connected controlling power supply system 31, and the ion current value is converted to the corresponding pressure value. For this current-to-pressure conversion, the electron current value as measured is used, but the gaseous molecules have a high density in the pressure range of above 10.sup.-3 Torr, which may produce positive ions more frequently. Those positive ions may be attracted by the filament having negative potential, therefore, collected at the filament. This may cause great errors when the electron current value is read. Measuring the accurate current value is therefore practically impossible. In order to avoid this, there is another vacuum gauging tube that is capable of measuring in a pressure range higher by two orders than the typical B-A ionization gauging tube, i.e., up to 1.times.10.sup.-1 Torr, wherein a supplemental or auxiliary electrode 24 is located adjacently to the filament 21 and has a lower potential relative to the filament, thereby reducing the possibility of the produced ions collecting at the filament. This vacuum gauging tube may be used for measuring the pressures during the sputtering process in particular (N. Ohsako "A new wide-range B-A gauge from UHV to 10.sup.-1 Torr", J. Vac. Sci. Technol., 20 (4), April 1982, pp. 1153-1155). The glass envelope also is coated or lined with the metal film 25 on the inner wall. Therefore, it prevents the inner wall from being charged by the ions that have been accumulated on the inner wall for the higher pressure measurement. Thus, it may serve to maintain the electrode elements inside at the constant potentials, respectively.
Typically, the conventional ionization gauging tube as described above has the geometrical and dimensional requirements for each of its constituent electrode elements, as set forth below. The glass envelope 11 usually has a cylindrical shape having a diameter of 50 mm to 60 mm and a length of 100 mm to 150 mm. The filament 21 is 40 mm to 50 mm long, and may be shaped like a straight needle, spiral coil, or hair pin. The grid 22 is made of a tungsten wire having a diameter of about one (1) mm, which is usually formed like a coil having a diameter of 15 mm to 25 mm, a length of 20 mm to 30 mm, and a winding pitch of about three (3) mm. Gases are degassed from the grid itself as well as constituent elements surrounding it, such as the filament or the inner wall, by heating the grid when the coil is energized across it to cause current to flow through it. Alternatively, the electron bombardment heating method may be employed instead of the coil grid heating method. In the ionization gauge employing this method, the grid is not coiled, and therefore is not energized across the current flow. In this method, the grid is operating in potential and electron current modes during the degassing stage which are normally different from those during the pressure measurement stage. For this reason, it has the disadvantage in that the pressure measurement cannot be performed during that time. Thus, the gauge employing this method only has the particular application where the ultra-low pressures are measured for experimental purposes. It is therefore not adequate for any other practical applications such as an industrial fabrication process.
The ion collector 23 is usually made of a tungsten wire having a diameter of about 0.2 mm and a length of 40 mm to 50 mm. The supplemental or auxiliary electrode is usually made of a metal banked like a semi-cylindrical shape having a diameter of 5 mm to 10 mm, and a length slightly greater than the filament.
It may be appreciated from the foregoing description that as one of the vacuum ionization gauging tube types, the Bayard-Alpert (B-A) type vacuum ionization gauging tube is an established technology, providing its performance, and has widely been used in measuring the vacuum pressures. According to the conventional type, all measuring constituent elements are accomodated in glass envelope which is liable to breakage when it is subject to any external force. It is therefore impossible to install it in any area where there is a risk of breaking the envelope. This imposes restrictions on the design of the vacuum system in which pressures are to be measured. Special care is also required during the maintenance service. Breakage may occur if the gauging tube is handled improperly or carelessly. For example, the envelope would easily be broken if a tool or part of the human body bumps against it inadvertently. If this occurs, the vacuum chamber would be exposes to the atmospheric pressures. To avoid this situation, any suitable covering may be provided for protecting the entire gauging tube. In this case, it is difficult to install the gauging tube within a limited space. For those years, the vacuum system has become more sophisticated and complicated, and contains many different components or elements within the limited space. In order to solve the above problem and thereby provide an ionization gauging tube that will never break, there is an ionization gauging tube whose measuring electrode elements are all accomodated in metal envelope, rather than glass one. For the metal type vacuum gauging tube, the radiant heat produced from the filament will irradiate on the metal envelope, raising the temperature of the metal envelope so as to degas more gases. This type ionization gauging tube usually includes cooling means for cooling the inner wall of the metal envelope. Without such cooling means, in the ultimate pressure range it is impossible to measure the pressure. In fact, the pressure measurement could not be made.